# A planet with mass m, and radius r .

1. Jun 6, 2005

### J_o_S

A planet with mass m, and radius r.....

what is the acceleration due to gravity on the surface of the planet?

a = Gm/r^2

correct?

I'm pretty sure I'm right, but I will sleep better if someone can confirm this

thanks

2. Jun 6, 2005

### whozum

Thats fine, just make sure m is the mass of the earth, not the object.

3. Jun 6, 2005

### J_o_S

so the mass of the planet (not earth) doesnt matter at all?

4. Jun 6, 2005

### whozum

You were asking about the acceleration of an object in earth's gravitational field, which is only dependant on the mass of the earth and the object's distance from the earth.

What is this other planet you mention? There are no planets near the surface of the earth.. atleast that i know of :surprised

5. Jun 7, 2005

### J_o_S

sorry, just want to make sure we're clear here. I think there might be a misunderstanding.

Lets call the planet: "Planet X"

I am given the mass and radius of Planet X. I want to find out the acceleration due to gravity on the surface of Planet X.

Now I thought I was supposed to use the mass of planet X, not earth, since earth doesnt have anything to do with the question

6. Jun 7, 2005

### whozum

Sorry, my misunderstanding. You are completely correct.

7. Jun 7, 2005

### ramollari

Put in a nutshell:

$$g = G\frac{M}{R^2}$$,

where g is the free fall acceleration at the surface of the planet,
M is the mass of the planet,
and R is the radius of the planet.